1. Field of the Invention
Electromagnetic radiation collector systems and, more particularly, solar radiation collector systems that include photovoltaic trackers, heliostats, solar furnaces, and point-focus collector systems.
2. Description of the Related Art
A typical related art concentrating solar collector system includes a concentrator having a suitable reflective surface which may be monolithic or formed from multiple individual mirrors, a receiver for absorbing the concentrated solar radiation, and associated support structures. Point-focus solar collector systems also require suitable primary and secondary tracking devices so that aligned mirrors can follow the apparent movement of the sun from dawn to dusk and through seasons. Such collector systems are arranged and constructed so that the sun's rays falling on reflective surfaces are directed into receivers to be utilized in any well known manner, such as heating a suitable circulating fluid which can be used to power an engine or be transported elsewhere for various uses, applying it directly to photovoltaic or other suitable direct energy conversion devices, utilizing it in a solar reactor for a variety of chemical processes, and the like. A receiver can be arranged to move with the concentrator or be fixed with respect to the moving concentrator assembly.
The related art is replete with a multitude of different designs of solar collector systems and tracking structures. Such related art systems have not been practicable and were typically complex, heavy and onerous to erect and service. Not a single one has met the commercial requirements of the marketplace. Current designs require subsidies for them to make economic sense, and these solar collectors typically take many years to replace the energy invested in materials and installation.
The related art has a variety of solar collectors that feature methods of lifting and lowering structures. For example, one type of technology can utilize a polar column hinged at the base and a pivoting equatorial mount to lift and lower a solar collector mounted between them. To support the concentrator and receiver, central structural devices with a shaft at each end have been suggested. An independent drive wheel or gear may rotate the concentrator to follow the sun. However, this type of technology does not accommodate solar seasonal motion. Another technology rotates the concentrator assembly around the receiver to track seasonal motion. Both these approaches would require very substantial central structures ending in significant shafts to accommodate the forces concentrated there into bearing devices at each end. Lacking adequate force distribution, wind and gravity loads would tend to twist and deform both kinds of structures.
The related art may utilize rings that are perpendicular to a right ascension, RA, axis that enable tracking in this dimension. Methods are also mentioned for tracking in declination. However, these related arts cannot transmit wind and gravity loads on the tracking structures into the foundations without great distortion nor do they illustrate alternative force distribution structures. None of the related art mentions associated foundation structures to raise and lower these solar collectors.
The related art illustrates ways to distribute gravity and wind forces involved in turntable-type structures that provide the primary tracking motion and that use integral accurate ring or rings to provide the secondary tracking motion. The related art may mention how to distribute loads in the secondary tracking structure. However, these forces have to be accommodated by the turntable arrangement that requires extensive foundations with a heavy accurate ring. One solution for field construction mentioned is to assemble the solar collector or heliostat in a building with required access to elevated assemblies and then to transport the completed unit to the site using heavy equipment. One of the related arts does not distribute forces in the secondary tracking structure but requires slender members that would rapidly fatigue, riding on four small rollers, and this approach would also require heavy equipment for erection. The very large offset overturning moments involved when high winds impact the elevated sail area of these concentrators would tend to pry both these approaches off turntable interfaces at ground level.
Multiple columns to raise and lower a concentrator have been suggested but this patent teaching would be primarily effective only near noon in tropical regions where the sun is nearly directly overhead. At other times and places, since the receiver is stationary, it would be difficult to get the concentrator shape appropriate for off-axis conditions. The reflective surface always faces skyward and, with multiple independent cable winches, would be extremely difficult to control. The associated receiver would require heavy equipment for installation. There seems to be no mechanism for stabilizing this kind of concentrator so that regions do not invert, like an inside-out umbrella, in high winds.
Another technology encountered major problems while trying to support a curved rim on two rollers. To prevent metal fatigue from concentrated forces at the two rollers, the curved structural member either has to be very robust and preferably heat treated like a railroad rail or the rollers have to be soft, like a rubber tire. In addition, not only were these large curved members difficult to form and transport, mounting an assembled solar collector on the columns required heavy equipment. Suspending the curved rim on multiple rollers mounted on a chain, in roller-chain style, between two columns did distribute the load but, mounted at the angle required for installations in the US, keeping the rollers tracking properly proved difficult. Although this method would work near the equator, where the curved rim is almost vertical, it is difficult to support the rollers working with RA axes at higher latitudes. It was also difficult, using this method of construction, to lift the structure off the ground.
Another technology which supports transducer devices on two polar columns provides supporting a third point on the tracking structure for providing stability and provides a way (tilting) to raise and lower the equipment. However such technology is used for supporting a heliostat and thus does not teach rotating an assembly to follow the sun, nor does it provide a way to operate in windy conditions.
Three issues have impeded deploying viable large solar collectors that harvest enough solar energy so they can become the preferred renewable energy resource:
1. Wind loads (managing forces on the structure, through drives and into the ground);
2. Hail, freezing rain, snow, frost and soiling (maintaining active area performance); and
3. High erection and maintenance costs (structure, interface and foundation approach).
Collecting significant amounts of solar energy requires a large area of reflector to redirect sunlight into a receiver. This reflector area acts like a sail and since they must follow the sun, concentrators at any orientation must transmit wind loads, coming from any direction, into the ground. To stay clean as possible, to prevent hail from damaging mirrors and to minimize exposure to wind, reflectors must face down when not operating. When facing skyward, hail may damage transducer elements (including both mirrors and photovoltaic approaches) and they accumulate dust, snow, freezing rain and frost. Because mirrors reflect and do not absorb radiation, sunlight warms them very slowly and ice deposits take a long time to melt, delaying operation and wasting energy. Utility-scale point-focus solar collectors (that have more than 800 square feet of reflector) and many smaller units all require heavy equipment both for erection and for repairing drives and bearings. Scheduling heavy equipment can be problematic. Cost to bring in a crane can exceed the value of energy a collector harvests in a year. Also, fossil carbon footprint accounting requires renewable energy equipment replace the resources used in manufacturing, construction and maintenance including fuel burned by heavy equipment. Erecting and repairing solar collectors should minimize using fossil fuel powered equipment.
Related art designs of heliostats and point-focus solar collectors utilize a variety of tall columns as in FIG. 1 through FIG. 3, or a turntable with elevated collector pivots as in FIG. 4. Each of these approaches requires a crane to lift and mount the collector assembly respectively between the columns, to the top of the single pedestal or between the elevated pivots of a turntable.
Referring now to the drawings, FIG. 1 and FIG. 2 illustrate two related art two-axis tracking, point focusing, solar collector systems 30 shown as equatorially oriented structures wherein systems have two motions: one, around the right ascension, RA, axis 49 parallel to the earth's polar axis and the other, around axes generally perpendicular to the RA axis. The solar radiation collector systems 30 include a main support structure 32 with a concentrator frame 36 for carrying one or more transducer elements. Reflective transducer elements 37 are supported and positioned either on transducer support members 43 connected with frame 36 in FIG. 1 or on linear transducer assemblies 41, seven in each of four quadrants, are illustrated in FIG. 2 to create a Fresnel reflective paraboloidal surface. These systems require suitable receivers 48 connected either to the main support structure 32 as shown in FIG. 1, or on dedicated support devices in front of the concentrator assembly 35 illustrated as two booms 40 in FIG. 2, so they move with the tracking structure. Receivers 48 are arranged for receiving the solar radiation directed from the transducer elements 37.
A desired primary tracking motion (right ascension) 49 rotates the main support structure with respect to the ground to counteract the rotating earth. The solar collector shown in FIG. 1 utilizes a friction drive roller 70 mounted on a motor-driven gear reducer of the right ascension drive 69 that directly turns the main support structure 32 around the RA axis 49. Shown in FIG. 2, the motor/gear reducer of the collector right ascension drive 69 that is fixed to the RA axis support 72 turns a small diameter sprocket that engages a drive wheel 71 attached to the main support structure 32.
To provide for the second motion (declination) 47 to adjust for the tilt of the earth responsible for seasonal changes in solar position, the transducer elements 37 are arranged and constructed to provide for the selective movement thereof with respect to the frames 36. That is, the reflective surface is of the dynamic Fresnel element type wherein the Fresnel mirrors, or other suitable transducer elements, are arranged to be selectively moved on axes generally perpendicular to the RA axis 49 to accommodate the seasonal variation of the solar position. Because the receiver 48 is typically on the optical axes of the two systems 30 illustrated in FIG. 1 and FIG. 2 only at equinox, the reflective surface, the mirror facets, or other transducer elements thereof must be adjusted to function properly at other times of the year. Conveniently, this can be achieved by moving facets individually, FIG. 1, by linear motion 68 of a member connecting rows of transducer elements 37 causing them to rotate or mounting the transducer elements on suitable transducer assemblies 41, FIG. 2, which are disposed within the frame 36 and provided with drive devices for selectively rotating them.
Although such related art dynamic Fresnel type concentrator solar collector systems have operated entirely satisfactorily with respect to the ability to concentrate and collect solar energy, they have remained too difficult to manufacture and erect to be entirely acceptable for many promising near term commercial applications. For example, such systems require either substantial columns to support large concentrators, or involve many foundations for both support columns and guy cables 53. The cables that anchor the columns to foundations and those mutually stabilizing the solar collector are difficult to see, especially in dim light, and avoiding them requires extraordinary care. Although the solar collector of FIG. 1 has three primary foundations: the two polar columns 59 and the equatorial foundation 52 with RA axis support 72, it also requires a foundation to anchor the tie down 64 that prevents wind from polar directions from overturning the assembly. The solar collector in FIG. 2 requires two polar foundations 50 and two equatorial foundations 52 to support the bifurcated polar structure 59 and RA axis support 72 along with six additional foundations to secure cable stays 53 to the ground.
FIG. 3 shows a typical point-focus solar collector mounted on a single pedestal 51. An azimuth drive rotates the tracking structure around a vertical axis 44 and an elevation drive pivots the concentrator and receiver around a horizontal axis 42. The center of gravity of the moving structure is ideally located where these axes intersect to minimize drive torque requirements. The concentrator frame 36 is mounted on one end of the main support structure 32, with the receiver 48 mounted on the other end. Assemblies of mirror facets 37 are attached to the concentrator frame 36. To stow the collector 30, the concentrator assembly 35 rotates up until it is partially inverted and the receiver 48 moves down to a limit set by interference between the single pedestal 51 and the main support structure 32. Both the azimuth and elevation drives that interface with the top of the central pedestal 51 are compact which requires an assembly with extraordinary strength and precision.
FIG. 4 shows a point focus solar collector mounted on a turntable 60 that has a central hub 38. The large diameter turntable structural member rides on rollers on top of many columns 61 which prevents snow and ice from interfering with operation. Wrapping a roller chain around the outside diameter of this turntable allows a stationary gear motor with a sprocket to effectively drive this solar collector system 30 around the vertical axis 44. Uplift is prevented by capturing the central hub 38 on the central column or providing uplift prevention devices on the column and roller assemblies 61. Two symmetrical main support structures 32 are each topped by bearings for suspending the concentrator frame 36 on the horizontal axis 42. A large diameter elevation drive support arc 55 allows a gear motor with a sprocket and chain arrangement similar to the azimuth drive with similar advantage. More than 180 degrees of motion with the concentrator assembly 35 facing past straight up to directly down is possible by simply extending the elevation support arc 55. Setting the shafts/bearings on the horizontal axis 42 so that the receiver 48 balances the concentrator assembly 35 minimizes loading and elevation drive power required.
Most two axis trackers for photovoltaic panels, heliostats and point focus solar collectors use a single pedestal. Although this lone foundation and column are simple, interface modules that mount on the top of the pedestal require dedicated castings or complex welded assemblies. These assemblies have to transmit large dead and live (wind) loads from the tracking structure to the pedestal and maintain tracking accuracy of the drives and bearings for decades. Thousands of pounds of wind acting on the wide sail area of the concentrator, say with a 36 foot diameter, is typically counteracted by a gear with a small radius, typically less than a foot, requiring precise (to minimize free motion) and very strong gear teeth. To replace these primary drive components and associated bearings typically requires removing the entire assembly from the pedestal. These issues limit single pedestal designs to around 800 square feet of active area.
To avoid concentrating forces through the small interface on top of a single pedestal, a variety of related art solar collectors use turntables that distribute support to multiple foundations. Turntables which do not use spokes would be quite small (concentrators less than 200 square feet) because the rings required for transmitting the wind forces involved in larger concentrators would be too heavy. Turntables which use spokes (supporting concentrators up to 5,400 square feet) are limited by the requirements to prevent uplift. If uplift (or overturning) forces are transmitted through the hub, very large forces are involved unless the length of the moment arm (the radius of the turntable ring) is also large which adds considerable weight to these assemblies. To prevent uplift through the connections between the turntable ring and multiple foundations for the rollers that support the ring and allow for rotation require accurately assembled mechanisms to capture the ring under all conditions.
Heliostats, solar furnaces, and point-focus solar collector systems utilize concentrators consisting of either a single monolithic reflector, one made of a parabolic array of mirrors or a Fresnel arrangement of mirrors. These systems follow the apparent daily and seasonal movement of the sun by two separate motions. Reflectors of both heliostats and solar furnaces direct sunlight to a stationary focal region. In a point focus solar collector system, the receiver typically moves along with the reflector. Point-focusing dish solar concentrators provide much higher optical and operating performance than any other type and have high temperature capability (3,000 degrees Fahrenheit and above), use minimal land, and are highly modular (power plant sizes from single kilowatts to many megawatts). Accordingly, such dish concentrator systems are very versatile and are adaptable to many markets for solar applications, particularly for generating electric power in both remote and community-scale installations, as well as providing industrial process heat, producing high value chemicals, making renewable fuels (hydrogen), and destroying toxic wastes.
Foundations, mechanical, electric, instrumentation, and communication interfaces and the tracking modules of point-focusing solar collector systems with related installation and quality control labor account for a large part of the construction effort. Because these subassemblies are similar for both large and small units, one can install a single large solar collector much more readily than many smaller ones. Structural issues begin to dominate this design approach for concentrators larger than 10,000 square feet, more than ten times the 800 square foot limit for single pedestals.
Accordingly, there remains a continuing need to improve solar collector systems for a wide range of commercial applications. Such solar radiation collector systems should:                be straightforward to manufacture, ship and install using common materials and indigenous facilities and not require expensive machinery or heavy equipment;        employ structures which are simple, strong, lightweight, and capable of supporting large concentrators and heavy receivers;        tolerate extreme weather conditions including severe winds, hail, freezing rain, snow, and the like without reducing performance;        allow ordinary people working together with hand tools to both manufacture and assemble models appropriate for harvesting and utilizing energy in their region; and        have axes of rotation pass through the center of gravity so it takes very little power to follow the sun or stow the equipment.        